This program seeks an understanding of the interaction of ultrasound with biological material in two broad perspectives: (1) Propagation of Sound in Tissue with particular emphasis on interfacial heating and nonlinear phenomena and (2) the Biological Effects of Ultrasound. In the latter work, both thermal and nonthermal mechanisms have been demonstrated. Plant roots provide an excellent material for studies of nonthermal effect e.g. growth inhibition, reduction in mitotic index. Particular emphasis is given to the physical mechanisms responsible for the effects. Mammalian tissues are to be tested for similar processes.